Subterranean deposits of natural resources such as gas, water, and crude oil are commonly recovered by drilling wellbores to tap subterranean formations or zones containing such deposits. Various tools are employed in drilling and preparing wellbores for the recovery of material therefrom such as logging tools having sensors for measuring various parameters downhole, data storage devices, flow control devices such as valves, transmitters, and receivers. Electrical power is generally required to power such downhole tools. The electrical power may be generated downhole with a power generator such as a turbine generator. However, power generators are relatively complex and often malfunction, resulting in the inability to use downwhole tools powered by such generators until the generators have been repaired or replaced. As such, using energy storage devices such as batteries, fuel cells, or capacitors to power downhole tools is considered a better alternative to the use of power generators.
As illustrated in FIG. 1, the minimum operating temperature of an energy storage device is a function of the rate of discharge of the energy storage device. The capacities of an energy storage device having a relatively low rate of discharge and one having a relatively high rate of discharge are plotted as a function of temperature in FIG. 1. The higher the discharge rate of an energy storage device, the higher the temperatures required for its operation. In particular, it requires higher temperatures to increase the mobility of ions in the electrolyte or the electrodes of the energy storage device. For example, energy storage devices that have solid electrolytes between the anode and the cathode, such as molten salt batteries or solid oxide fuel cells, have relatively high minimum operating temperatures.
Unfortunately, ambient temperatures in the wellbore are often lower than the minimum operating temperatures of energy storage devices utilized therein. As a result, those devices fail to provide downhole tools with sufficient power to operate at full capacity. This problem is commonly encountered when an energy storage device is used at shallow depths in a wellbore where downhole temperatures are lowest. A need therefore exists to develop a method for improving the operability of an energy storage device that has a minimum operating temperature above ambient temperatures in a wellbore in which the device is located.